1. Field of the Invention
The present invention relates to the improvements of a temperature-sensing, variable-speed fan drive with a fluid coupling, incorporated in a cooling system of an internal combustion engine.
2. Description of the Prior Art
FIG. 9 shows a typical temperature-sensing, variable-speed viscous fan drive with a small fluid coupling partly filled with a special silicone oil. The conventional fan drive (or a fan coupling device) 101 includes a driven housing 104 rotatably supported on a drive shaft 102 by way of a ball bearing 103, a partition plate 108 dividing the internal space of the housing 104 into a fluid reservoir chamber 105 and a working chamber 106, a communication port 107 formed in the partition plate 108 in such a manner as to intercommunicate the reservoir chamber 105 and the working chamber 106, a rotor 109 fixedly connected to the front end of the drive shaft 102 and housed within the working chamber 106, a torque-transmitting viscous-coupling portion 110 transmitting torque from the rotor 109 to the housing 104, and a valve mechanism 111 capable of regulating the amount of working fluid flowing from the reservoir chamber 105 via the communication port 107 to the working chamber 106 by adjusting or controlling the opening or size of the communication port 107 depending on the atmospheric temperature in the circumference of the housing 104. The drive shaft is driven by the engine crankshaft of an internal combustion engine. As seen in FIG. 9, the housing 104 is constructed by a housing body 121 rotatably supported on the drive-shaft front end through the ball bearing 103, and a front cover portion 122 tightly fitted to the front face of the housing 104. The cover portion 122 has a substantially circular recessed portion 123 formed in its inner peripheral portion to define the reservoir chamber 105. Also, the cover portion 122 has a ring-shaped flanged portion 125 formed in its outer peripheral portion to define a working-fluid return passage 124 therein. The outermost peripheral portion of the housing 104, consisting of the housing body 121 and the cover portion 121, is formed with a plurality of cooling-fan mounting bolt holes for mounting the cooling fan on the housing 104 by means of bolts. The viscous-coupling portion 110 consists of two sets of concentric labyrinth portions 127 and 128 alternately fixed to the flanged portion 125 of the housing and the rotor 109, with the viscous fluid between them. As is generally known, the previously-noted valve mechanism 111 includes a temperature-sensing bimetallic coil, a valve operating shaft, and a valve portion. When engine cooling requirements are low, such as during cool-weather, intermediate-speed operation, the communication port 107 is fully closed by the valve mechanism 111 to cut off the flow of working fluid from the reservoir chamber 105 to the working chamber 106 in order to block the flow of working fluid into the viscous-coupling portion 110. Thus, the working fluid is withdrawn from the torque-transmitting, viscous-coupling portion 110 via the return passage 124 into the reservoir chamber 105. Thus, so less power or torque passes through and the fan speed drops to the minimum. In contrast to the above, when engine cooling requirements are high, such during high temperature, high-speed operation, the communication port 107 is fully opened by the valve mechanism 111 to allow the flow of working fluid from the reservoir chamber 105 to the working chamber 106. Thus, more working fluid is fed via the communication port 107 toward within the torque-transmitting, fluid coupling portion 110. More power or torque passes through the coupling portion 110 and thus the fan speed increases, so as to effectively cool an engine cooling-system radiator. One such temperature-sensing, variable-speed viscous fan drive with a small fluid coupling has been disclosed in Japanese Utility-Model Provisional Publication Nos. 57-204491 and 3-77825. On automotive vehicles with an air conditioning system, a sole temperature-sensing, variable-speed fan coupling device is often used for cooling at least two stacked heat exchangers, namely a cooling-system radiator, and an air-conditioner condenser usually located in front of the radiator. On cars with two stacked heat exchangers, there is the following problem. If the air conditioning system comes into operation under a particular condition where engine cooling requirements are high, as in the summer, and the engine begins to run or is idling, it is difficult to adequately cool the air-conditioner condenser, for the reasons set out below.
That is, when the air conditioning system comes into operation under the previously-noted particular condition, air temperature in the circumference of the air-conditioner condenser rises, and thus refrigerant pressure begins to rise. Suppose the atmospheric temperature in the circumference of the cooling-fan housing does not yet rise. In such a case, the cooling fan never rotates. The condenser cannot be cooled satisfactorily, thus resulting in reduction in the air conditioning system efficiency (the heat exchanging efficiency). To avoid this (to ensure a required airflow through the condenser), it is possible to add auxiliary fans. This induces a more complicated construction of the system, and increases production costs.